Electro-mechanical frequency comparator utilizing cams for timing



3,204,181 REQUENGY COMPARATOR J. A. COOPER ET `AL UTILIZING CAMS FOR TIMING Filed Jan. 25, 1963 ELECTRO-MECHANICAL F Aug. 31, 1965 United States Patent O 3,2%81 ELECTRQ-MECHANCAL FREQUENCY (KEMP/lll- ATG?. UTllLlZilrltG CAMS FR 'IlltdlNG Jameson A. Cooper, Pleasantville, and Richard Say/lor,

Morrisey, NSY., asslgnors to General Precision, lne., a corporation of Delaware Filed dan. 23, i963, Ser. No. 253,396 6 Clair-ns. (Cl. 324-79) This invention relates to frequency comparators and rore particularly to a comparator for simultaneously comparing a voltage of unknown frequency with a reference voltage of known frequency and for indicating if :the unknown frequency stays within a predetermined percentage, plus or minus, of the reference frequency over a substantial preset length of time.

Many devices have een employed for performing this type of frequency comparison. However, they all employ the technique of frequency division until the periods of the two frequencies equal the time over which .the comparison is to taire place. Since the time is substantial, approximately three or more minutes, these devices are complicated as wel-l as bulky.

The comparator herein disclosed utilizes frequency division, but does not divide the frequency to the extent set forth above and in fact stops far short of that point. The divided frequencies, of approximately ten to fifteen cycles per second, are applied to identical frequency responsive motors which have their outputs connected to the inputs of a differential which supplies a shaft rotation equal to their average frequency difference over the measurement period. A plurality of cams operated by the output shaft of the differential perform switching functions which when combined with a logic switching circuit provide indications of a successful comparison or an unsuccessful comparison.

@ne object of this invention is to provide a novel frequency comparator for simultaneously comparing a voltage of unknown frequency with a voltage of known frequency over a substantial preset length of time and for indicating when the two frequencies deviate, over the time of the measurement, from each other by more than a predetermined amount.

Another object of the invention is to provide a frequency comparator as set forth above which is accurate, reliable and inexpensive to manufacture.

A further object of the invention is to provide a frequency comparator of the class described which occupies a small volume of space.

The foregoing and other objects and advantages of the invention will become more apparent from a consideration of the speciiicaton and drawing wherein one embodiment of the invention is `described and shown in detail for illustration purposes only.

The single ligure is a block and schematic diagram of p a novel frequency comparator constructed according to the invention.

A reference voltage of known frequency, from a source not shown, is applied to an input terminal l1 while a voltage of unknown frequency, from a source not shown, is applied to another input terminal lf2. Terminal 11 is connected to a first frequency dividing circuit lll by a normally closed set of contacts Kl-1 operated by a relay K1, the connection of which, will be described in detail later.

Throughout the drawing contacts disassociated from their operating relay windings are shown by spaced parallel lines. if the contacts are normally closed, that is they are closed when their relay Winding is ile-energized, a connecting line at an angle is utilized to indicate that they are closed. lf, on the other hand the contacts are dfhll Patented Aug. 3l, i965 normally open, that is they are open when their ralay winding is de-energized and closed only when the Winding is energized, no connecting line is used. ln addition, contacts operated by the K1 relay will all be numbered K1 plus an additional numeral to distinguish one contact from the other. With this system it is possible to readily identify that relay winding which operate-s `any given contact.

input terminal l2 is connected to a second frequency dividing circuit 15 by series-connected normally closed contacts K55-ll and Kll-Z. Normally open contacts ld-2 connect terminal lll to the common junction of contacts lid-1 and Kil-2 while normally open contacts K11-3 connected the `terminal directly to the input of frequency dividing circuit l5. The input of frequency dividing circuit ld is connected to ground by normally open contacts Kl-t.

With the circuit switching yarrangement described above terminals 1l and 12 are connected to dividers 10i and i5, respectively, when windings K1 and K5 are deenergized. rThis condition pervais during a comparison. After a comparison is completed a reset mode is entered into, which requires the energization of windings K1 and Ke', and in this mode the reference voltage only is applied to frequency divider f5. How reset is accomplished will be described in detail later. When winding K5 is energized and winding lil tie-energized, the reference voltage is applied to frequency dividers 1dand 15S and terminal 12. is disconnected by contacts KS-l which open. This mode is the self-test mode where the known frequency is appplied to both dividers. Here again a detailed description will be deferred till later.

Frequency dividers 1L! and 15 are identical and each may include a plurality of binary countdown circuits which divide the input frequency until it reaches a predetermined frequency. ln one embodiment which was constructed and successfully operated the input frequency was reduced in the dividers to 121/2 cycles per second. The reduced frequency from divide-rs lll and 1S is applied to stepping motors 16 and 17, respectively, which provide 25 steps per second when their input frequencies are 121/2 cycles per second. The motors selected for the above embodiment provide l5-degree steps which resulted in 24 steps per revolution of the output shaft of the motor. The particular frequencies selected are not critical and may be varied to suit different applications of the comparator. However, if changes are made they will affect all the other components and compensating changes will be necessary to retain the appropriate scale factors. Motors 16 and 17 are connected to a direct current power supply i9 by normally closed contacts iid-1 operated by relay winding K3. Power supply 19 provides the power necessary to operate the motors in response to the inputs from dividers 1d and 15.

The output shafts of motors 16 and 17 are connected to the opposite sides of a mechanical differential Ztl which has an output shaft 2l. Output shaft 21 provides a rotation equal to the difference of the rotations of the input shafts connected to motors 16 and 1.7 and is connected by a gear reducing unit 23 to a cam shaft 24 which drives a reset cam 25, a fail cam 26, and an error cam 27. rlihe output shaft of motor ld, in addition to being connected to differential Ztl, is connected by a second gear reducing unit 23 to a second cam shaft 29 which drives a timer cam 3).

Timer cam 3l? can only malte one revolution for a complete test cycle; therefore, if the test is to taire 3 minutes and the output of frequency divider 14- is lili/2 cps., gear reducing unit 2S must have a ratio of 187.5:1. However, it is not essential that the test be limited to exactly 3 minutes, nor is it necessary that the exact output frequency of divider lf:- be maintained at precisely 121/2 cps.; therefore, any convenient, substantially similar ratio may ybe selected for gear reducing unit 23 since all that can vary in this instance is the length of time over which the comparison is made and as was previously pointed out this is not critical.

lt was arbitrarily decided that a 4% error between the reference and the unknown frequency would result in one complete rotation of shaft 2d and the three cams associated therewith. Therefore a 7.511 ratio was used in gear reduction unit 23. This ratio must be held close if the scale factor selected is to be maintained since any varie.- tion would result in a change in scale factor an-d the accuracy of the device would not hold. The 121/2 c.p.s. frequency of divider M will not, if allowed to vary, atect the scale factor since, regardless of the frequency, one revolution of the timer always includes the same number of steps of reference motor l5, which in the case of a 3-minute test cycle with the ratio set forth above for unit 23, equals 4,500 steps.

Error cam 27 has a notch 27N which extends for 126 with the scale factor employed; reset cam 25' has a very narrow notch 25N, which is, in the reset position, which will be defined later, physically aligned with the center of notch 27N of cam Z7; and fail cam 26 has a datent 26D, 180 from notch 25N. With this arrangement any error exceeding .7% in one direction would cause the cam follower 27E associated with cam 27 to come out of notch 27N thus indicating an error in excess of .7%. It is possible, however, that the error might exceed 3.3%, in which case follower 27E would be back in notch 27N which would indicate a valid comparison. Therefore, fail cam 26 and detent 26D are provided to supply a permanent indication whenever the detent 26D actuates the follower 26E associated with fail cam 26 by a latching circuit which will be described later. lVhen reset cam 25 occupies the reset position, a follower 25E associated with the cam resets in notch 25N to indicate that all the cams are in the reset position.

The overall accuracy of the comparator is excellent since the errors introduced by the components are small. A basic error of plus or minus one count may always be preset and equals 0.022%. The only other significant errors are introduced by the cams and are directly related to cam alignment and to the tolerance on the Width of notch 25N of reset cam 25. ln one device constructed the total cumulative error, including the 0.622% error above, was 0.056% which is as good or better than the accuracy attainable with prior comparators.

Each of the cams operates an armature which makes and b'eaks one of two circuits depending on the position of its associated cam. Cam follower 25E is connected to an armature 32 which engages a Contact 32A when the follower is out of notch 25N and a contact 32B when the follower 25E is in notch 25N. Cain follower 26E is connected to an armature 33 which engages a contact 33A. when the follower is displaced by detent 26D and a contact 33B at all other times. Cam follower 27E is connected to armature 3d which engages a contact 34A when the follower is in notch 27N and a Contact 34B at all other times. A cam follower Stil? associated with timer cam 30 is illustrated within a notch 36N on cam 3i?. The follower is connected to an armature 35 which engages a contact 35A as long as follower 39E is within notch 30N and a contact 35B at all other times.

The comparator utilizes three lamps for indicating the IN PROCESS condition and a successful or unsuccessful comparison. The IN PROCESS lamp 37 is connected between ground and contact 35B. The GO lamp 33 which indicates a successful comparison is connected between ground and contact 34A while the NO G0 lamp 39 which indicates an unsuccessful comparison is connected between ground and contact 34B. Lamp 39 is also provided with an alternate connection which will be dcscribed later.

Cperation of the comparator is controlled by three momentary contact push-button switches fill, 42 and i3 labeled START, SELF-TEST and RESET, respectively. SELETEST switch is a dual-contact switch and has two normally open contacts A and B. RESET switch Lt3 is also a dual-contact switch but has one normally open Contact A and one normally closed contact B. START switch t1 is of the normally open single contact type. The connection of the above switch contacts will be dcscribed in due course as the description proceeds.

The comparator in addition employs eight relays having operating windings Kl; through KS inclusive. Each of the relays has a plurality of normally closed and normally open contacts operated by the relays. The nomenclature or numbering system described earlier will be used in the description of the contact connection which follows.

Windings Kl through K7 each have one end connected directly to ground while one end of winding K3 is connccted to ground through normally closed contacts 43B of the push-button RESET switch 43. Each of the windings has its end remote from ground connected to voltage source 19 by one or more switching paths any one of which when closed energizes the relay it connects to source fr?.

Winding Kl is connected to source 19 by four different paths. Each of the paths includes normally closed contacts KS-l. The first and second paths both include in addition contact 35A and wiper which are directly connected to source 19. The rst path also includes contacts while the second includes normally open contacts KZ-tl. Paths three and four each include in addition to contacts KS4, contacts 42B and normally open contacts 2K5-3. The third path also includes normally closed contacts KIs-2 connected to source 1.9, while the fourth path includes normally closed contacts Ki-l connected to source 19. Winding K2 is connected to the common junction of contacts littl and 431A and includes the four paths described above with the exception of contacts Kit-1l. lt should be noted however that the path including contacts KZ-l is a latching path and cannot energize the winding. It can only hold the winding in the energized state once it is energized by appropriate contact closures in one of the other three paths.

Winding K3 is connected to source Il@ by two different paths each of which includes normally closed contacts K'-ll. Both include in addition Contact 35A and armature 35, -previously described. The first path also includes normally closed contacts K22 while the second includes normally open contacts iid-2. Winding Kd utilizes the same four paths as winding Kl described above with eX- ception of contacts ld-3l. In the case of winding K4 normally open contacts ld-2 are employed in place of normally closed contacts Htl-ll, previously described. Thus it is seen that windings Kt and K4 cannot be simultaneously energized.

Relay winding K5 may be initially energized by any one of two paths each of which include contacts -t-ZB and 35A and armature 3-5, all previously described. The first in addition includes contacts 43A while the second includes contacts KZ-Ts, all previously described. Two latching paths are also provided. Both have contacts K5-3 in common. The first has in addition contacts liti-ll while the second has contacts LEG-2, all previously described.

Winding K6 can only be energized when fail cam 26 rotates, in either direction, a suicient amount to cause detent 26D to displace follower 26E and thus close contact 33A. Since this may only be a momentary closure, latching contacts lidare connected in parallel with contact 33A and armature This parallel circuit is connectcd to source t9 by four paths all of which include previously described contacts. The rst and second paths utilize contacts lili-l and 1(3-2, respectively, while the third and fourth paths each include contacts ICS-3, 42B, 35A and armature 35. ln addition the third path indescribed, is necessary before another test cycle may start. This is true even after a self-test cycle in which none of the cams move since forced reset is employed.

The forced reset is necesary to insure that the cam followers, especially follower F, enter their respective notches from the same direction at all times. if this is done the possible error is substantially reduced. Forced reset is initiated by contacts 43B since these contacts open and de-energize winding K3 which simulates, as far as the comparator is concerned, the condition present when follower 25F is out of notch 25N. Thus a reset must proceed until winding K8 is again energized when the follower ZSF enters notch 25N.

While only one embodiment of the invention has been shown and described in detail for illustration purposes, applicants wish it clearly understood that the invention is not limited to the specic details disclosed.

What is claimed is:

l. A frequency comparison circuit for providing indications when two frequencies compared fall within a predetermined percentage of each other comprising,

a first frequency dividing means responsive to a reference frequency for dividing the frequency by a predetermined amount,

a second frequency dividing means responsive to an unknown frequency for dividing,y the unknown frequency by the same predetermined amount,

iirst movable means responsive to the first frequency dividing means producing a mechanical movement corresponding in amplitude to the frequency supplied by said first frequency dividing means,

second movable means responsive to the second frequency dividing means producing a mechanical movement corresponding in amplitude to the frequency supplied by said second frequency dividing means,

means responsive to the mechanical movement of said first and second movable means producing a bidirectional mechanical difference movement having an amplitude and direction corresponding to the amplitude and sign, respectively, of the difference between the mechanical movements of said first and second movable means,

a first displaceable element responsive to said difference movement,

first bistable means coupled thereto whereby said first bistable means resides in its first stable state when the first displaceable element occupies a predetermined position and its second stable state when said rst displaceable element is located in any other position,

a second displaceable element responsive to said difference movement,

second bistable means coupled thereto including actuating means whereby said second bistable means resides in its first stable state when said actuating means is located between two predetermined fixed points located on said second displaceable element and is in its second stable state at all other times,

a third displaceable element responsive to said difference movement,

third bistable means coupled thereto whereby said third bistable means resides in its first stable state as long as a predetermined point on said third displaceable element does not attain a selected position with respect to said third bistable means and shifts to and remains in its second stable state Whenever said selected position is attained, and

means responsive to said first, second and third bistable means for providing a first indication during a comparison when said first bistable means resides in its second stable state, a second indication at the end of a comparison provided said second and third bistable means are both in their first bistable states, and a third indication at the end of a comparison CII whenever either of said second and third bistable means occupies its second stable state.

2. A frequency comparison circuit as set forth in claim l in which the first, second and third displaceable elements each include a unique cam surface which undergoes movement in response to the difference movement,

and each of said first, second and third bistable means includes a cam follower urged into engagement with its respective cam surface and a two-channel switching means operated in response to its associated cam follower.

3. A frequency comparison circuit as set forth in claim in which the means responsive to the mechanical movements from said first and second movable means is a mechanical differential having a first and second input connected to the first and second movable means, respectively, and an output which produces a mechanical movenient equal in amplitude to the difference of the amplitude of the movements of the two inputs.

45. A frequency comparison circuit as set forth in claim in which the first, second and third displaceable elements each include a unique cam surface which undergoes movement in response to the difference movement,

and each of said first, second and third bistable means includes a cam follower urged into engagement with its respective cam surface and a two-channel switching means operated in response to its associated cam follower.

5. A frequency comparison circuit for providing indications when two frequencies which are being compared fall within predetermined percentages of each other cornprising,

a first frequency dividing means responsive to a reference frequency for dividing the frequency by a predetermined amount,

a second frequency dividing means responsive to an unknown frequency for dividing the unknown frequency by the same predetermined amount,

first movable means responsive to the first frequency dividing means producing a mechanical movement corresponding in amplitude to the frequency supplied by the said first frequency dividing means,

second movable means responsive to the second frequency dividing means producing `a mechanical movement corresponding in amplitude to the frequency supplied by the said second frequency dividing means,

means responsive to the mechanical movements from said first and second movable means producing a bidirectional mechanical difference movement having an amplitude and direction corresponding to the amplitude and sign, respectively, of the difference between the mechanical movements of the said first and second movable means,

means responsive to the amplitude and direction of the last said means for providing a rst or second indication when the said difference movement, in any one given direction, is above or below a predetermined percentage of the total mechanical movement of the said first movable means,

and means responsive to the total movement of said first movable means for controlling, as a function of said total movement, said means for providing the first and second indication to terminate the comparison at the end of a predetermined interval.

6. A frequency comparison circuit as set forth in claim S in which the means responsive to the mechanical movements from said first and second movable means is a mechanical differential having a lirst and second input connected to the first and second movable means, respectively, and an output which provides a mechanical movement equal in amplitude to the difference of the amplitude of the movements of the two inputs.

(References on following page) 9 10 References Cited by the Examiner 2,503,105 4/50 Freas 324-85 UNITED STATES PATENTS FOREIGN PATENTS 2,176,742 10/ 39 La Pierre 324-79 706,298 3/54 Great Britain. 2,334,863 11/43 Canetta et al. 324-79 5 2,424,333 7/47 Korman '324. 79 WALTER L. CARLSON, Primary Exammer. 

1. A FREQUENCY COMPARISON CIRCUIT FOR PROVIDING INDICATIONS WHEN TWO FREQUENCIES COMPARED FALL WITHIN A PREDETERMIN ED PERCENTAGE OF EACH OTHER COMPRISING, A FIRST FREQUENCY DIVIDING MEANS RESPONSIVE TO A REFERENCE FREQUENCY FOR DIVIDING THE FREQUENCY BY A PREDETEMINED AMOUNT, A SECOND FREQUENCY DIVIDING MEANS RESPONSIVE TO AN UNKNOWN FREQUENCY FOR DIVIDING THE UNKNOWN FREQUENCY BY THE SAME PREDETERMINED AMOUNT, FIRST MOVABLE MEANS RESPONSIVE TO THE FIRST FREQUENCY DIVIDING MEANS PRODUCING A MECHANICAL MOVEMENT CORRESPONDING IN AMPLITUDE TO THE FREQUENCY SUPPLIED BY SAID FIRST FREQUENCY DIVIDING MEANS, SECOND MOVABLE MEANS RESPONSIVE TO THE SECOND FREQUENCY DIVIDING MEANS PRODUCING A MECHANICAL MOVEMENT CORRESPONDING IN AMPLITUDE TO THE FREQUENCY SUPPLIED BY SAID SECOND FREQUENCY DIVIDING MEANS, MEANS RESPONSIVE TO THE MECHANICAL MOVEMENT OF SAID FIRST AND SECOND MOVABLE MEANS PRODUCINGA BIDIRECTIONAL MECHANICAL DIFFERENCE MOVEMENT HAVING AN AMPLITUDE AND DIRECTION CORRESPONDING TO THE AMPLITUDE AND SIGN, RESPECTIVELY, OF THE DIFFERNCE BETWEEN THE MECHANICAL MOVEMENTS OF SAID FIRST AND SECOND MOVABLE MEANS, A FIRST DISPLACEABLE ELEMENT RESPONSIVE TO SAID DIFFERENCE MOVEMENT, FIRST BISTABLE MEANS COUPLED THERETO WHEREBY SAID FIRST BISTABLE MEANS RESIDES IN ITS FIRST STABLE STATE WHEN THE FIRST DISPLACEABLE ELEMENT OCCUPIES A PREDETERMINED POSITION AND ITS SECOND STABLE STATE WHEN SAID FIRST DISPLACEABLE ELEMENT IS LOCATED IN ANY OTHER POSITION, A SECOND DISPLACEABLE ELEMENT RESPONSIVE TO SAID DIFFERENCE MOVEMENT, SECOND BISTABLE MEANS COUPLED THERETO INCLUDING ACTUATING MEANS WHEREBY SAID SECOND BISTABLE MEANS RESIDES IN ITS FIRST STABLE STATE WHEN SAID ACTUATING MEANS IS LOCATED BETWEEN TWO PREDETERMINED FIXED POINTS LOCATED ON SAID SECOND DIPLACEABLE ELEMENT AND IS IN ITS SECOND STABLE STATE AT ALL OTHER TIMES, A THIRD DISPLACEABLE ELEMENT RESPONSIVE TO SAID DIFFERENCE MOVEMENT, THIRD BISTABLE MEANS COUPLED THERETO WHEREBY SAID THIRD BISTABLE MEANS RESIDES IN ITS FIRST STABLE STATE AS ALONG AS A PREDETERMINED POINT ON SAID THIRD DISPLACEABLE ELEMENT DOES NOT ATTAIN A SELECTED POSITION WITH RESPECT TO SAID THIRD BISTABLE MEANS AND SHIFTS TO AND REMAINS IN ITS SECOND STABLE STATE WHENEVER SAID SELECTED POSITION IS ATTAINED, AND MEANS RESPONSIVE TO SAID FIRST, SECOND AND THIRD BISTABLE MEANS FOR PROVIDING A FIRST INDICATION DURNG A COMPARISION WHEN SAID FIRST BISTABLE MEANS RESIDES IN ITS SECOND STABLE STATE, A SECOND INDICASTION AT THE END OF SA COMPARISION PROVIDED SAID SECONDAND THIRD BISTABLE MEANS ARE BOTH IN THEIR FIRST BISTABLE STATES, AND A THIRD INDICATION AT THE END OF A COMPARISON WHENEVER EITHER OF SAID SECOND AND THIRD BISTABLE MEANS OCCUPIES ITS SECOND STABLE STATE. 